Benzene is a naturally-occurring constituent of crude oil and a constituent of many petroleum products. Its average concentration in motor gasoline in the United States is on the order of 1%. Benzene has received much attention from regulatory agencies because it has been classified as a known human carcinogen (EPA classification, Group A) and causes blood disorders (leukemia) in workers exposed to high concentrations.
With the toxicity factor in view, the US Environmental Protection Agency and regulatory agencies in other countries and regions, specifically, the European Union, have set rules for some time regulating the levels of benzene permitted in motor gasolines. Benzene in conventional gasoline is currently controlled indirectly by regulations that limit benzene via exhaust toxics tied, in most cases, to a refinery's Mobile Source Air Toxics (MSAT) Phase 1 baseline but new regulations proposed by the US Environmental Protection Agency, will subject all US gasoline to benzene restrictions at levels far lower than those currently applicable only to reformulated gasoline (RFG) which constitutes about 30-35% of the US gasoline pool. RFG was held to a benzene limit of less than 1.0 vol % but under MSAT Phase 2, regulations are expected to reduce benzene in all US gasoline to an annual average 0.62 vol % starting in 2011; in addition, refiners must also meet a maximum average benzene standard of 1.3 vol % beginning in 2012. The current limit on benzene in regions subject to the Euro III and Euro IV standards is 1 vol %. In addition, all Canadian and Japanese gasoline is subject to the 1.0 vol % limit. Reduction of the benzene content in all US gasoline to 0.6 vol % under EPA MSAT Phase 2 will make it more difficult for foreign suppliers to provide gasoline for the US market. Imported gasoline has supplied more than 10% of US summer gasoline in the past few years and has helped significantly to stabilize US gasoline supplies during periods of high demand. The effect of MSAT Phase 2 on U.S. refiners is, of course, more direct and it is likely that benzene reduction solutions that worked with RFG may be insufficient to meet the new specifications.
For most refiners, benzene reduction will be a compliance issue. A refiner must choose and implement a solution that provides the best value in the highly competitive gasoline market. Solutions will be refinery-specific and determined by the current refinery configuration, the type of reforming unit, the amount of benzene contributed from other blendstocks, amount of oxygenate blended, and access to markets that allow trading of intermediate streams, e.g. benzene to petrochemical plants. A refiner's first step in meeting the new benzene requirement will be to review the benzene contributions to the pool from each source. Next, the operating windows and roles for the existing assets in light of the new regulations will require consideration, factoring in the effects from alternative crudes that may be processed, future refinery expansions, and any changes in FCC unit operations that affect benzene. Once these preliminary steps are complete, the refiner must evaluate all the possible solutions and determine if one offers better economics and flexibility than another.
While removal of benzene from aromatic gasoline blend stocks be required, removal from reformate streams is likely to become the more important factor under the new regulatory regime since a limit of 1 vol % in the gasoline pool, as under current regulations, enables the level of benzene in FCC naphtha, which is about 0.5-1.3 vol % depending on FCC operation, catalyst, and feed, to be given less weight. At a maximum 0.6 vol % benzene in the gasoline pool, however, the contribution from FCC naphtha may require refiners to look at more expensive benzene-reduction solutions. Coupled with this is the need to maximize the size of the gasoline pool, which inevitably requires a higher level of conversion, much of which is provided by the FCC unit.
The main source of benzene in most gasolines is reformate and most current benzene reduction solutions focus on preventing benzene formation in the reforming unit by removing benzene precursors from the reforming unit feed. This solution, however, has the potential drawback of reducing the amount of hydrogen produced in the reformer and so, of reducing the amount of hydrogen produced for other refinery processes such as desulfurization, hydrocracking, FCC feed hydrotreating which themselves can contribute to the quality not only of the refinery gasoline pool but also to the quality of other products and to the cleanliness of the environment.
Extraction of benzene from the reformate, either for petrochemical production or for chemical conversion followed by return of the remainder to the gasoline pool provides a net hydrogen balance of zero but in this case, the volume of the refinery gasoline pool is reduced by the removal of the benzene. The removal of benzene by extraction may also result in a decrease in product octane quality since benzene and other single ring aromatics make a positive contribution to product octane (MON Blending Numbers are 91 for benzene, 112 for toluene, 124 for m-xylene, 124 for isopropylbenzene and 129 for propylbenzene). The retention of aromatics, although not in the form of benzene but rather the less toxic alkylaromatics, is therefore desirable from the viewpoint of good product quality, engine operation and, in addition, the improved fuel economy resulting from the higher volumetric energy content of the aromatics.
Certain processes have been proposed for converting the benzene in aromatics-containing refinery streams to the less toxic alkylaromatics such as toluene and ethyl benzene which in themselves are desirable as high octane blend components. One process of this type was the Mobil Benzene Reduction (MBR) Process which, like the closely related MOG Process, used a fluidized zeolite catalyst in a riser reactor to alkylate benzene in reformate to from alkylaromatics such as toluene. The MBR and MOG processes are described in U.S. Pat. Nos. 4,827,069; 4,950,387; 4,992,607 and 4,746,762. The fluid bed MBR Process uses a shape selective, metallosilicate catalyst, preferably ZSM-5, to convert benzene to alkylaromatics using olefins from sources such as FCC or coker fuel gas, excess LPG or light FCC naphtha. Normally, the MBR Process has relied upon light olefin as alkylating agent for benzene to produce alkylaromatics, principally in the C7-C8 range. Benzene is converted, and light olefin is also upgraded to gasoline concurrent with an increase in octane value. Conversion of light FCC naphtha olefins also leads to substantial reduction of gasoline olefin content and vapor pressure as well as a decrease in MON sensitivity.
Like the MOG Process, however, the fluid bed MBR Process required considerable capital expenditure, a factor which did not favor its widespread application in times of tight refining margins. The MBR process also used higher temperatures and C5+ yields and octane ratings could in certain cases be deleteriously affected, another factor which has not favored widespread utilization. Other refinery processes have also been proposed to deal with the problems of excess refinery olefins and gasoline; processes of this kind have often functioned by the alkylation of benzene with olefins or other alkylating agents such as methanol. Exemplary processes of this kind are described in U.S. Pat. Nos. 4,950,823; 4,975,179; 5,414,172; 5,545,788; 5,336,820; 5,491,270 and 5,865,986.
Co-pending applications published as US 2006/0194998, US 2006/0194996 and US 2004/0194995, disclosed a simple, economical, fixed bed process for converting benzene to alkylaromatics with light refinery olefins especially ethylene, propylene and butene. The process was notable for its ability to: upgrade C2 and C3 olefin from fuel gas to high octane blending gasoline; increase flexibility in refinery operation to control benzene content in the gasoline blending pool; avoid octane loss and hydrogen consumption associated with alternative reformer feed tailoring and benzene saturation technologies; remove benzene from the refinery gasoline pool without diverting the benzene to other uses; and allow refineries with benzene problems to feed the C6 components (low blending octane values) to the reformer, increasing both the hydrogen production from the reformer while retaining the octane contribution from the high octane alkylaromatics. In operation, an increase of 1-10 numbers of (R+M)/2 may be achieved, depending on the feed composition, benzene conversion and endpoint specification.
Because the main objective of the process is to reduce the benzene content of the feed stream, achieving a high level of benzene conversion is important; complete or near complete conversion of the benzene is an obvious goal. This objective has, however, proved difficult to achieve in view of competing and conflicting process and equipment requirements.
The benzene conversion in the reformate alkylation process needs to achieve a high level in order to meet the gasoline composition specifications. Desirably, the conversion should be at least 90%, preferably at least 95% or even higher in order to maximize the extent of benzene removal. At the same time, the product should be held to meet refinery gasoline pool blending specifications, most notably the end point or T90 specifications which, in the United States, should not exceed 225° C. (437° F.) on the end point and 185° C. (about 365° F.) for T90 (T90 values in this specification in accordance with ASTM D 86). To comply with more restrictive specification such as CARB (California Air Resources Board), a lower figure closer to 145° C. (about 293° F.) will be required for T90. What this means in terms of process design is that highly polyalkylated benzenes are undesirable because of their effect on T90 and, possibly for their other adverse effects in the gasoline pool: 1,3,5 tri-isopropyl benzene and 1,2,4,5-tetraisopropylbenzene each have, for example, a melting point of 118° C. and excessive amounts in the product can lead to crystallization in cold weather. Thus, a balance needs to be struck between the octane boost resulting from alkylation and the resulting increase in melting point and boiling point. Allied to this is also the desirability of conserving refinery olefins which may have other uses if available and of minimizing hydrogen consumption.